The invention relates to improved modules and methods and apparatus for removably retaining the modules on a rigid underlying support surface, to improved screening decks for screening particulate material, and more particularly to support frames which can be removed and replaced easily as circumstances require. Another aspect of the invention is directed to a means for connecting a screening module to an underlying support frame. Various screening decks and screening elements are known which are intended for the same general purposes. An example of a crowned screening element is shown in U.S. Pat. No. 4,120,784. The screening element is crowned in order to pretension the element for greater strength. However, crowned elements are difficult to remove and install and wear unevenly.
Various ways are known for connecting a screening element to an underlying frame to form a screening deck. Some methods require spacers or adapter bars between the screening element and the frame. An example of such an arrangement is shown in U.S. Pat. No. 4,409,099. That patent shows screening elements supported only at their corners on spacer elements so that the screening element is spaced above the frame, ostensibly to increase the effective screening area. The intermediate spacer elements add considerably to the cost and complexity, and increase the height of the overall screening apparatus. In addition, specially formed frame members are required to reduce abrasion of frame members by the sifted particulate material. That requirement represents an expensive departure from standard practices in the industry.
Another screening apparatus that employs specialized components intermediate the screening elements and the support frame is shown in U.S. Pat. No. 4,219,412. There, a molded plastic frame is adapted to receive individual screening elements. The plastic frame, in turn, is connected to the frame. This combination leads to several disadvantages similar to the device of the U.S. '099 patent mentioned above. The '412 patent does not disclose whether or how individual screening modules might be removed and replaced.
Similarly, U.S. Pat. No. 4,661,245 shows a screening system that incorporates clamping bars which receive the individual screening elements. The clamping bars, in turn, are bolted onto the frame. The resulting structure is unduly complex, expensive, increases the height of the deck and suffers other shortcomings described below.
Several methods also are known for connecting flat screening elements directly to a supporting frame. One method is the use of a depending hook or skirt member integrally formed in the outlet side of the screening element. An example is shown in U.S. Pat. No. 3,980,555. According to that invention, depending hooks of abutting screening elements are inserted into a frame member through a common mounting hole. The hooks extend out of the mounting hole on the underside of the frame and engage the frame member to hold the screening elements in place. The resulting screening deck has a substantially contiguous inlet surface that provides no access to the mounting hooks for disconnecting an element which requires replacement. Accordingly, such arrangements require a user to climb under a screening deck and cut the depending hooks off of a module to be replaced. Or, more commonly, a user will pry the element off from the top (inlet side) and destroy it in the process. Either way, the module is rendered useless; a result that is acceptable for modules which are worn out, but wasteful if the module is being replaced merely to change sieve size.
Another attachment method is shown in U.S. Pat. No. 4,219,412, discussed above. There, individual screening elements are connected to an intermediate supporting frame by integrally formed depending hook-shaped members, similar to those disclosed in the U.S. '555 patent. Here, however, the depending hook members are inaccessible even from the outlet side of the screen, as they are enclosed within elongate U-shape channels. The '412 patent does not disclose any way of removing an individual screening element for replacement, short of disassembling the screening deck.
Another method of connecting screening elements to an underlying frame is to employ securing pins as suggested, for example, in U.S. Pat. No. 4,141,821. According to that invention, abutting screening elements each include hollow tubular protrusions extending through a common mounting aperture formed in the frame so as to define a hollow tube. A securing pin is inserted into the tube from the inlet side of the deck to spread the protrusions apart beneath the aperture and thereby engage the frame.
The securing pin taught in the U.S. Pat. No. 4,141,821 patent is difficult to pry out of the tube. The pin perhaps could be driven out from the outlet side of the screen. In any event, once removed, the pin is loose and therefore subject to loss. This is a decided risk in the context of screening machines, as it often is impractical or impossible to retrieve an errant securing pin. A similar securing pin is employed in the apparatus disclosed in U.S. Pat. No. 4,409,099.
An improved screening system developed by the common assignee of this patent application is provided in U.S. Pat. No. 5,049,262, and is incorporated herein by reference. The screening system includes a rigid frame and a plurality of screening modules. A lock pin is provided which detachably connects the screening modules to the rigid frame. The lock pin is designed so that it remains connected to the frame while allowing removal of the screening modules from the frame, and positioning of a replacement screening module on the frame.
Yet another apparatus for removably mounting screening elements is disclosed in U.S. Pat. No. 4,670,136. That patent discloses a two-piece elongate clamping element for clamping the edge of a plastic module or wire mesh screen. The apparatus includes a first, lower plastic profile fastened to the frame and arranged to engage under the edges of at least two mutually adjacent screen elements. A second, upper profile is detachably fastened over the first lower profile for retaining the screen elements clamped in place. Removal of a screening element requires removal of the upper plastic profile, thereby subjecting it to risk of loss like the securing pins of the U.S. '821 patent and the like. Additionally, the clamping structures of the U.S. '136 patent extend substantially beyond the edges of the frame, thereby reducing the effective screening area and subjecting the clamping structure itself to excessive wear from abrasion by particulate material being screened.
Another class of modular screen system are represented by European Patent No. 167,999 ("EP '999") and DE 3606-854 ("DE '854"). More specifically, EP '999 is directed to a modular screen system which includes an assembly for attaching sieve elements 2 to metal support girder 12. The attachment assembly comprises upper and lower profile sections 6 and 14, respectively, and a protective ledge 10. The upper profile section 6 engages the entire side portion of screen elements 2, while the lower profile section 14 is connected within a continuous slot in metal support girder 12. The upper profile can have a bolt-shape cross-sectional configuration. The protective ledge 10 covers the entire top surface of the support girder 12 to prevent wear from abrasive material passing through the sieve elements 2. Since the upper profile section 6 is in locking engagement with the entire extent of the side of the screen element 2, it is extremely difficult and time consuming for users to remove and replace individual damaged screen modules. Furthermore, this removal process often results in unwanted destruction adjacent undamaged screen modules. Also, longitudinal positioning is a problem since there is a minimum impediment to movement of the screen element 2 along the profile section 6.
Accordingly, the need remains for a system for effectively and efficiently screening particulate material which provides for fast, convenient and nondestructive replacement of screening modules.